Voting opened at 4 a.m. and closed at 5 p.m. Results weren't expected to be known until over the weekend.

Eric Williams, local president, wrote in a statement on the union website:

"We entered into negotiations with the full intent to receive a fair and equitable contract for our members and their families. Unfortunately, the San Francisco Municipal Transportation Agency had a different premise on negotiating fairly. The city devalued our service as they proposed unreasonable takeaways in wages and benefits."

Muni workers, like all San Francisco city employees, are prohibited from striking. Under Proposition G, approved by city voters in 2010, if union members reject the proposed contract, it goes to an arbitrator who cannot rule against Muni management's proposals unless the union can prove that its interests outweigh "the public interest in efficient and reliable transit."

Union officials have complained that the standard is too high to overcome, but legal challenges to Prop. G have been largely unsuccessful. The arbitration procedure, he wrote, "is a lopsided and unfair process. The membership must take a stand, which will be communicated through your vote."

Despite that statement, local Vice President Andrew Simmons said the union is not encouraging a vote against the proposal.

"It is completely up to the membership how they vote," he said Friday afternoon.

Rumors of a sickout

Muni service was noticeably delayed on some lines Friday morning, and rumors of a sickout or work slowdown to protest the contract circulated. But MTA spokesman Paul Rose said the delays were the result of an operator shortage that has plagued Muni on a variety of lines for months.

"We are aware of the rumors and will continue to monitor service," he said. "We are confident we will be able to provide service to our riders and we believe our operators know how important their work is to the city."